1. Field
The invention relates to digital television (DTV) signals for over-the-air broadcasting, transmitters for such broadcast DTV signals, receivers for such broadcast DTV signals and in particular those items as designed for implementing a system of broadcasting robust data to mobile/handheld receivers, collectively referred to as “M/H” receivers.
2. Related Art
The Advanced Television Systems Committee (ATSC) published a Digital Television Standard in 1995 as Document A/53, hereinafter referred to simply as “A/53” for sake of brevity. Annex D of A/53 titled “RF/Transmission Systems Characteristics” is particularly incorporated by reference into this specification. In the beginning years of the twenty-first century, efforts were made to provide for more robust transmission of data over broadcast DTV channels without unduly disrupting the operation of so-called “legacy” DTV receivers already in the field. In 2009, these efforts culminated in a candidate ATSC standard directed to broadcasting digital television and digital data to M/H receivers being drafted. This candidate standard, referred to as “A/153”, is incorporated by reference within this specification.
A/153 prescribes forward-error-correction coding of data transmitted to M/H receivers, which FEC coding comprises transversal Reed-Solomon (TRS) coding combined with lateral cyclic-redundancy-check (CRC) codes to locate byte errors for the TRS coding. This FEC coding helps overcome temporary fading in which received signal strength momentarily falls below that needed for successful reception. The strongest TRS codes prescribed by A/153 can overcome such drop-outs in received signal strength that are as long as four tenths of a second.
Another known technique for overcoming temporary fading is iterative diversity. Iterative diversity can also overcome certain types of intermittent radio-frequency (RF) interference. Communications systems provide for iterative diversity of received signals by transmitting a composite signal composed of two component content-representative signals, one of which is delayed with respect to the other. The composite signal is broadcast to one or more receivers through a communications channel. At a receiver, delayed response to the initially transmitted component content-representative signal supplied from a buffer memory is contemporaneous in time with the finally transmitted component content-representative signal. Under normal conditions, the receiver detects and reproduces the content of the finally transmitted signal as soon as it is received. However, if a drop-out in received signal strength occurs, then the receiver detects and reproduces the content of the initially transmitted signal as read from buffer memory. If the delay period and the associated delay buffer are large enough, then fairly long drop-outs in received signal strength can be overcome. This capability not only requires a several fold increase in the amount of memory required in a receiver; it halves the effective code rate of the transmission. However, overcoming drop-outs as a long as a few seconds are envisioned is soon being feasible.
Thomson, Inc. proposed forms of iterative diversity its engineers called “staggercasting” for use in robust portions of 8-VSB transmissions. Thomson, Inc. has advocated iterative diversity in which the earlier and final transmissions of the same data are combined in the “transport” layer of the receiver. The “transport” layer of the receiver is subsequent to the “physical” layer of the receiver, which recovers transportstream (TS) packets from the robust portions of 8-VSB transmissions. TS packets from the earlier one of the iterated transmissions replace missing TS packets in the later one of the iterated transmissions in staggercasting. For a brief time Thomson, Inc. and Micronas GmbH representatives within the ATSC took a position that the earlier and final transmissions of the same data could be advantageously combined in the physical layer of the receiver, rather in its transport layer, but later withdrew from that position. Thomson and Micronas jointly proposed a concatenation of outer block coding with inner ⅔ trellis coding per 8-VSB for each component transmission, pointing out that earlier and final transmissions of the same coded data could be combined along the lines used in digital audio broadcasting (DAB).